Full credit goes to avbrand.com this technique. I've made some minor tweaks so it will work if you system is configured with a proxy. Remember you're using a web service (https://tohtml.com) to perform the highlighting -- so if the code is really sensitive this may not be for you.

Enable Macros

Click 'File', followed by'Options'

Find 'Trust Center', then 'Trust Center Settings'

Select the 'Macro Settings' tab

Select 'Enable all macros', and click OK.

Again, this is not ideal, I'll try and follow up with how to self sign your macro

Enable Developer Mode

Right-click the Ribbon and select 'Customize the Ribbon'

In the list on the right side, underneath Main Tabs, there should be checkbox labeled Developer. Check box if not already checked.

Click OK to close the window.

Open Visual Basic editor

Click the 'Developer' tab. To the far right of File

Click the 'Visual Basic' button

Add reference to necessary libraries

Click the 'Tools' menu, then 'References'

Click 'Browse', then type 'FM20.DLL' and press OK

This is the Microsoft Forms 2.0 Library

Click OK

Time to code

Expand the Project pane on the left and double click 'ThisOutlookSession'

A blank code window should appear, if not follow the directions on where to paste the contents below.

The first portion of the above defines functions for each language you might want to highlight -- feel free to add more from the list on http://tohtml.com. Use the values of the entries in the 'Type' combo box (i.e. look at the page source).

For example this would add a function for Cobol:

Public Sub PasteCobol()
PasteCode "cobol"

To finish up create buttons for the email drafting window:

Open an email compose or reply window.

I have had mixed results when the e-mail in embedded in the Outlook window -- "Pop-out" the composition window first.

Right-click the Ribbon and select 'Customize the Ribbon'

Click 'New Group' in the lower right

Rename it to 'Paste Code'

On the Left side change the dropdown from 'Popular Commands' to 'Macros'